Just remember, the Earth is not a sphere!
If you assume it is, you will get lost enough to cause real trouble. To a first approximation, you can use a spheroid.
For instance of the errors, the vertical line at any point on a perfect spheroid (other than the equator or the pole) does not pass through the centre of the Earth. The Earth isn't even a *perfect* spheroid!
And the WAY in which it departs from a sphere or a spheroid is very important - different countries use different spheroids and projections for very real reasons.